One of the problems with a spring assembly of the kind wherein a plurality of spring elements are positioned in transversely and longitudinally-spaced relation to each other between a rigid base frame and a rigid grid frame comprising a border wire and transversely and longitudinally-extending grid wires connected at their ends to the border wire and at their crossings to each other is that pressure brought to bear on the grid frame tends to bend the border wires downwardly so that the border wire sags. In order to minimize this, bent wire reinforcements are sometimes used to support the border wire relative to the base frame. Adding such reinforcing wires requires added operations in manufacture and increases the cost without materially improving the structure. It is the purpose of this invention to employ the unique characteristics of the bent wire spring units disclosed in my pending application in such a way as to impart tension to the border wire, hence, to prevent sagging without interposing reinforcing springs along the sides and ends.